1. Field of the Invention
The present invention relates generally to power supplies, and more specifically to multi-stage AC/DC power supplies and methods for directly converting rectified alternating current (AC) input to produce a low voltage direct current (DC) output with low input current harmonic distortion.
2. Background of the Invention
Switching power converters are typically used in a wide variety of applications. Off-line AC/DC converter applications (for operation from an AC power line source) are a common example. Power factor correction is often incorporated in such power converters and typically takes one of two forms: a separate power factor correction stage at the input of the power converter or a power converter designed with power factor correction as part of the overall control function of the power converter.
Recently, the development of high-efficiency semiconductor light sources has generated a demand for low power high-efficiency off-line converters. Use of high-efficiency semiconductor light sources in traffic control, industrial and household applications is on the rise. Compatible and efficient power converters for supplying power from an AC line to a low voltage output at a relatively low power level are therefore a necessity. It is further necessary in some semiconductor light source applications to provide power factor correction (PFC) and a low level of AC input harmonic current, even in low power applications. In some applications, since a multitude of power supplies for the semiconductor light sources operate in parallel off of an AC line power source, power factor correction and reduction of AC line harmonic currents are a necessity to provide high efficiency and a low level of line interference.
Two-stage converters combining a power-factor correction (PFC) stage with a down converter stage are typically used when high efficiency and low input current harmonics are required. However, typical two-stage converters are costly for low power applications. Single-stage power factor corrected power supplies have been proposed, but for low voltage DC outputs they are generally restricted to transformer-coupled applications due to the step-up characteristic of a boost input stage that provides the PFC functionality. An attempt to operate these power converters as direct-coupled converters (as opposed to transformer coupled converters) would require the second stage down converter to operate at a very low duty cycle in order to produce a low output voltage, leading to inefficient operation.
Quadratic power converters for providing low voltage outputs from a wide range of input voltage were proposed by D. Maksimovic and S. Cuk in the article xe2x80x9cSwitching Converter with Wide DC Conversion Rangexe2x80x9d, May 1989 proceedings of the HFPC and also in xe2x80x9cSwitching Converters with Wide DC Conversion Rangexe2x80x9d published in the Institute of Electrical and Electronic Engineer""s (IEEE) Transactions on Power Electronics on January 1991. The topologies use a single switch to control cascaded buck and buck-boost stages. However, PFC functionality cannot be provided using these topologies. In order to provide good power factor performance, the input buck-boost stage must operate in the discontinuous conduction mode (DCM) with a nearly fixed duty ratio. Operation in continuous conduction mode defeats the PFC operation, as the input current is no longer a function of the duty ratio. Also, a low frequency filter (energy storage device) must be incorporated in the power supply topology to provide regulated DC output throughout the AC line cycle.
Therefore, it would be desirable to provide low output voltage cascaded converters that may be directly coupled to an AC line without a need for a step-down transformer while maintaining power factor correction and a low level of input current harmonics.
The above objective of providing low output voltage cascaded converters that may be direct-coupled to the AC line, while maintaining power factor correction and a low level of input current harmonics is achieved in a method and apparatus. The apparatus includes a rectifier for rectifying an AC voltage input to produce a rectified DC output, a first converter stage for correcting input power factor and converting the rectified power output to an intermediate DC voltage lower than a maximum level of the rectified DC output, and a second converter stage for converting the intermediate DC voltage to an output DC voltage. The first converter stage includes an input circuit comprising a first inductor series coupled with a first diode, a switch for coupling in series with a first diode. The series connected circuit is connected to said rectifier so that the first converter section is prevented from conducting energy into the rectifier when the voltage at the rectifier output falls below a threshold.
The foregoing and other objectives, features, and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiments of the invention, as illustrated in the accompanying drawings.